Docs: Add deprecation warning to Online Docs mentioning `gen_statem`

[rafaelbiten]

We've updated the README.md to recommend gen_statem as an alternative to this Connection module, but users visiting the module via the Online Documentation may miss that note, so it may be a good idea to have that note in there too.

This PR adds that same note as a deprecation warning to the @moduledoc itself.

This is what the updated docs look like with this update: